When grinding discs for either of the above purposes, it is important that the outside diameter of the disc is finished to a high level of accuracy and often to a particular cross-sectional form. In the case of memory disk a circular opening is also required again to an accurately controlled diameter and circularity. In the case of silicon wafers, registration in later manufacturing steps requires registration devices to be formed around the periphery of the disc such as the formation of flats and notches.
Conventionally edge grinding and polishing machines have incorporated linear slideways for all axes. Whether incorporating re-circulating rolling element bearings or air bearings, all such axes share a common failing when grinding and polishing brittle material namely they permit significant relative motion between grinding wheel and component. This arises from the need to provide for orthogonal movements of grinding wheels usually obtained by stacking one linear axis above another. This motion requires the use of hard wearing grinding wheels to minimise loss of form due to wear, but such wheels tend to produce poor quality surfaces with deep damage.
When grinding silicon wafers, the depth of sub-surface damage arising during grinding should be minimised as a wafer must be delivered with zero damage for use in subsequent manufacturing steps. When sub-surface damage has arisen, this means a post grinding step of acid etching before polishing. Both processes are expensive, and the less the damage the shorter the polishing time that is needed.
Normally the edge of a wafer or disc workpiece is machined so as to create a generally triangular or trapezoidal edge profile to the disc. In order to achieve this the surface of the grinding wheel is formed in a complementary manner so that as the wheel is engaged with the workpiece the complementary form is generated around the edge thereof.
The edge profile will only be generated accurately if the complementary form of the grinding wheel does not alter. Clearly as the grinding wheel wears, this profile will change and periodically such wheels have to be re-formed using a suitable forming wheel for resin bond wheels or spark erosion for metal bond wheels. Hitherto at least the initial forming of the complementary grinding wheel profile has been done "off machine" since hard wearing grinding wheels have tended to be used.